Download Paleozoic Life

Paleozoic
Life:
Invertebrates
Burgess
Shale
Paleozoic Life
• Geology drove biology
– evolution and plate tectonics
• Evolution affected by
– the opening and closing of ocean basins
– transgressions and regressions of epeiric seas
– formation of mountains
– changing positions of the continents
Tremendous Biologic Change
• Dramatic biologic change in Paleozoic
– appearance of skeletonized animals
– marine invertebrates adaptive radiation and
evolution
• diversification and extinction
• culminating at the end of the Paleozoic Era in the greatest
mass extinction in Earth history
Archaeooides, an enigmatic spherical
Cambrian fossil from the Mackenzie
Mountains, Northwest Territories,
Canada – specimen is several
millimeters in size
Cambrian Explosion
• Beginning of the Paleozoic Era
– animals with skeletons appeared abruptly in the fossil
record
• Sudden and rapid appearance of new animals in the
fossil record
– rapid, however, only in the context of geologic time
– millions of years during the Early Cambrian Period
• How many million?
Not a Recent Discovery
• Early geologists observed sudden appearance
of skeletonized animals
• Charles Darwin addressed this problem in On
the Origin of Species
– such an event was difficult to reconcile with his newly
expounded evolutionary theory
http://ethomas.web.wesleyan.edu/scie639/EXP
L.gif
Sharp Contrast
• The sudden appearance of shelled animals
– contrasts sharply with the biota living during the
preceding Proterozoic Eon
• Up until the evolution of the Ediacaran fauna
– Earth was populated primarily by single-celled
organisms
• The Ediacaran fauna
• consists primarily of
multicelled softbodied organisms
Restoration of the Ediacaran Environment
Time Between
• Long time period assumed to have existed
– between the extinction of the Ediacaran fauna
– and the evolution of the first Cambrian fossils
• That gap has narrowed in recent years
– with the discovery of more fossils
• known Proterozoic fossil assemblages continue right to
the base of the Cambrian
– recent work from Namibia indicates that Ediacaranlike fossils are even present above the first
occurrence of Cambrian index fossils
Triggering Mechanism
• Hypotheses about triggers:
– critical threshhold
• both biological and geological
• Environmental trigger (Snowball Earth):
– Earth was glaciated one or more times during the
Proterozoic
– followed by global warming during the Cambrian
• Evolutionary trigger:
– appearance of Hox genes
• (how do we know this?)
Snowball Earth
• Shallow seas formed as supercontinents
broke up
• organic matter could be produced rapidly and- decomposition was restricted
• atmospheric O2 goes up; CO2 and CH4 go
down;
• Usually when glaciers form, their formation
stops CO2 consumption by rock weathering
• thwarted by equatorial continents
• eventually volcanoes outgassed enough CO2
to melt the Snowball
Hox genes
• all metazoans (animals except sponges
+ algae) have the same genetic controls
on body organization
• Hox genes: master switch in body
pattern creation (‘put a leg here and an
eye there’)
• these genes first appeared in the
Cambrian
When Hox genes mutate...
Skeletons
(1) protection against ultraviolet radiation,
allowing animals to move into shallower
waters
(2) prevent drying out in an intertidal
environment
(3) protection against predators
– recent evidence of actual fossils of
predators
• and specimens of damaged prey
• indicates that the impact of predation during the
Cambrian was great
Cambrian Ecology
• Reconstruction of Anamalocaris
– a predator from the Early and Middle Cambrian
– about 45 cm long and probably fed on trilobites
– gripping appendages presumably carried food to its
mouth
Wounded Trilobite
• Wounds to the body of the trilobite
Olenellus robsonensis
– wounds have healed suggesting predation
Cambrian Marine Community
• Body plans
• Most documented evolutionary experiments
• Almost all the major invertebrate phyla evolved
during the Cambrian Period
– many were represented by only a few species
• Majority of Cambrian skeletonized life:
– trilobites
– brachiopods
– archaeocyathids
Cambrian Marine Community
• Floating jellyfish, swimming arthropods,
benthonic sponges, and scavenging trilobites
Trilobites
• Most conspicuous element of the Cambrian
marine invertebrate community
– about half of the total fauna
– benthonic
– mobile
– sediment-deposit feeders
• Appeared in the Early Cambrian, and rapidly
diversified,
– reached their maximum diversity in the Late
Cambrian,
– and then suffered mass extinctions near the end of
the Cambrian, never fully recovered
Trilobite Extinctions
• No consensus on what caused the trilobite
extinctions
– plate tectonics -> changed ecological niches
– increased competition
– rise in predators
• A cooling of the seas may have played a role
– particularly for the extinctions that took place at the
end of the Ordovician Period
http://www.nmsu.edu/~geology/zuhl/trilobite.jpg
Cambrian Brachiopods
• Cambrian brachiopods
– not abundant until the Ordovician Period
http://www.palaeontology.geo.uu.se/Mainp
ages/Brachiopoda/brach
Ordovician Brachiopod
Archaeocyathids
• Archaeocyathids, an extinct group of sponges
– benthonic sessile suspension feeders
– constructed reeflike structures
• The rest of the Cambrian fauna consisted of
representatives of the other major phyla,
– including many organisms that were short-lived
evolutionary experiments
http://www.carleton.ca/~tpatters/teaching/intro/
cambrian/cambrianex16.html
Cambrian Reeflike Structure
• Restoration of a Cambrian reeflike
structure built by archeocyathids
Burgess Shale Soft-Bodied Fossils
• 1909, Charles D. Walcott
• Smithsonian Institution,
– first soft-bodied fossils from the Burgess Shale
– a discovery of immense importance in deciphering
the early history of life
• Walcott and his collecting party split open
blocks of shale
– yielding the impressions of a number of soft-bodied
organisms
– beautifully preserved on bedding planes
Burgess Shale
• Presented a much more complete picture of a
Middle Cambrian community
• What conditions led to the remarkable
preservation of the Burgess Shale fauna?
Reason for the Preservation
• Animals preserved in the Burgess Shale
– lived in and on mud banks
– that formed along the top of a steep submarine
escarpment
• Periodically, this unstable area would slump
– slide down the escarpment as a turbidity current
– mud and animals carried with it were deposited in a
deep-water anaerobic environment
Rare Preservation: Burgess
Shale
• Ottoia, a carnivorous worm
Rare Preservation: Burgess Shale
• Wiwaxia, a scaly
armored sluglike
animal
Rare Preservation: Burgess
Shale
• Hallucigenia, a velvet worm
Rare Preservation: Burgess
Shale
• Waptia, an anthropod
How Many Phyla arose during the Cambrian?
• At the center of that debate are the Burgess
Shale fossils
• Early hypothesis: most Burgess Shale
organisms are part of existing phyla
• Competing hypothesis: Cambrian had many
more phyla
Burgess Body Plans
• Highly diverse
– Gould: more diverse than ever before
– Evolutionary experimentation
Ordovician Marine Community
• A major transgression that began during the
Middle Ordovician (Tippecanoe sequence)
– resulted in the most widespread inundation of the
N. A. craton
• This vast epeiric sea, which experienced a
uniformly warm climate during this time
– opened numerous new marine habitats
– soon filled by a variety of organisms
Striking Changes in Ordovician
• Both sedimentation patterns and fauna
underwent striking changes
• Ordovician was characterized by the adaptive
radiation of many animal phyla
• brachiopods
• Bryozoans
• corals
– with a consequent dramatic increase in the diversity
of the total shelly fauna
Middle Ordovician Seafloor Fauna
• Cephalopods, crinoids, colonial corals,
trilobites, and brachiopods
Brachiopods
•Brachiopods
—present since the Cambrian
—began a period of major diversification
—in the shallow-water marine environment during the
Ordovician
Graptolites
• Excellent guide fossils
– especially abundant
– most graptolites were planktonic
– most individual species existed for less than a
million years
• Due to the fragile nature of their organic
skeleton
– most commonly found in black shales
Conodonts
– well-known small toothlike fossils
– composed of the mineral apatite
• (calcium phosphate)
– the same mineral that composes bone
Mass Extinctions
• End of the Ordovician a time of mass
extinctions in the marine realm
– > 100 families of invertebrates became extinct
• What caused such an event?
– another massive glaciation?
Silurian and Devonian Marine
Communities
• Ordovician extinction -> rediversification,
recovery of:
– brachiopods, bryozoans, gastropods, bivalves,
corals, crinoids, and graptolites
Massive Reef Builders
• Silurian and Devonian were times of major reef
building
• While most of the Silurian radiations of
invertebrates represented repopulating of
niches
• Organic reef builders diversified in new ways
– building massive reefs
– larger than any produced during the Cambrian or
Ordovician
Middle Devonian Reef
• corals, cephalopods, trilobites,
crinoids, and brachiopods
Ammonoids
• Excellent guide
fossils !
– for the Devonian
through Cretaceous
periods
– with their distinctive
suture patterns
– short stratigraphic
ranges
– widespread
distribution
Another Mass Extinction
• Another mass extinction occurred near the
end of the Devonian
– worldwide near-total collapse of the massive reef
communities
– were most extensive in the marine realm
Another Mass Extinction
• The tropical groups were most severely
affected
– in contrast, the polar communities were seemingly
little affected
• Apparently, an episode of global cooling
– was largely responsible for the extinctions near the
end of the Devonian
– WHY?
Actors in Extinctions
• During such a cooling, the disappearance of
tropical conditions
– would have had a severe effect on reef and other
warm-water organisms
• Cool-water species, on the other hand, could
have simply migrated toward the equator
• The closing of the Iapetus Ocean and the
orogenic events of the Late Devonian
– undoubtedly also played a role in these extinctions
– by reducing the area of shallow shelf environments
where many marine invertebrates lived
Remember the connection between the tectonic and faunal changes!
Carboniferous and Permian
Marine Communities
• The Carboniferous invertebrate marine
community (Mississipian and
Pennsylvanian)
– responded to the Late Devonian extinctions
– in much the same way as the Silurian
invertebrate marine community responded to
the Late Ordovician extinctions
– that is, by renewed adaptive radiation and
rediversification
Mississippian Marine Life
– blastoids
– crinoids
– lacy bryozoans
– brachiopods
– small corals
Restricted Permian
Marine Faunas
• The Permian invertebrate marine faunas
resembled Carboniferous faunas
– not as widely distributed
– restricted size of the shallow seas on the
cratons
– reduced shelf space along the continental
margins
Permian Period
• Paleogeography
of North
America during
the Permian
Period
Permian Patch-Reef Community
• From Glass Mountains of West
Texas
– algae, productid brachiopods,
cephalopods, sponges, and
corals
The Permian Marine
Invertebrate Extinction Event
• Greatest recorded mass-extinction event
– occurred at the end of the Permian Period
• Before the Permian ended
– roughly 50% of all marine invertebrate families
– and about 90% of all marine invertebrate species
became extinct
Phanerozoic Diversity
• Diversity of marine invertebrate and vertebrate
families
– 3 episodes
of
Paleozoic
mass
extinction
are visible
– with the
greatest
occurring at
the end of
the
Permian
Period
Permian Mass Extinction
• Potential causes:
– (1) a meteorite impact (like the end of the
Cretaceous Period)
– (2) a widespread marine regression resulting from
glacial conditions
– (3) volcanic eruptions -> CO2 -> warming ->
collapsed ocean circulation -> anoxia -> euxinia
Permian Mass Extinction
• It appears that the Permian mass extinction
– took place over an 8-million-year interval at the end
of the Permian Period
– which would seemingly rule out a meteorite impact:
see reading for essay three for more discussion of
this
Biota Dramatically Changed
• Regardless of the ultimate cause of the
Permian mass extinctions,
– the fact is that Earth's biota was dramatically
changed
• Triassic marine faunas were of low
diversity
– but the surviving species tended to be
abundant
– and widely distributed around the world